The present invention relates to an industrially useful method for production of a monodispersed sparingly water-soluble salt with very uniform intragranular composition and ultrafine grains of sparingly water-soluble salt, and a production apparatus therefor.
In recent years, fine grains have drawn attention in various fields. Among such fine grains are photographic emulsions, which have relatively long been known.
Silver halide grains for photographic emulsion are usually produced by reaction of an aqueous solution of silver salt and an aqueous solution of halide in an aqueous solution of colloid in a reactor. Known methods include the single jet method, in which an aqueous solution of silver salt is added to a protective colloid solution such as gelatin containing an aqueous solution of halide in a reactor while stirring the latter solution, and the double jet method, in which an aqueous solution of halide and an aqueous solution of silver salt are separately and simultaneously added to an aqueous solution of gelatin in a reactor. The double jet method offers silver halide grains with narrower grain size distribution and allows free alteration of the halide composition of grains according to their growth.
Also, the speed of growth of silver halide grains is known to vary widely depending on the silver ion (halide ion) concentration, silver halide solvent concentration, grain suspension density, grain size and other factors of the reaction solutions. In particular, locally uneven distribution of solution density, which occurs in the stage of transition to uniform concentration of the aqueous solution of silver salt and aqueous solution of halide added to the reactor, leads to unevenness in the composition and grain size of the resulting silver halide emulsion as a result of changes in silver halide grain growing speed depending on the concentration of each solution. To eliminate this after-effect, it is necessary to rapidly and uniformly mix the aqueous solution of silver salt and aqueous solution of halide supplied to the aqueous solution of colloid in the reactor in reacting them. In the conventional method, in which an aqueous solution of halide and an aqueous solution of silver salt are added onto the surface of an aqueous solution of colloid in the reactor, halogen ion and silver ion concentrations become relatively high somewhere in the vicinity of the site of addition of each reaction solution, which hampers formation of uniform silver halide grains. Known means of improving this local unevenness of concentration include the methods disclosed in U.S. Pat. Nos. 3,415,650 and 3,692,283 and British Patent No. 1,323,464. These methods are characterized in that an oblate barrel-like mixing vessel, having a discharge slit in the central maximum diameter portion thereof and an opening in at least one of the upper and lower portions thereof, is immersed in an aqueous solution of colloid filled in a reactor so that the axis of rotation thereof is in the vertical direction, an aqueous solution of halide and an aqueous solution of silver salt are supplied from the upper or lower opening to the mixing vessel rotating at high speed through a supply tube, rapidly mixed and reacted therein, and the silver halide grains formed are discharged into the aqueous solution of colloid in the reactor by the centrifugal force produced by rotation of the mixing vessel.
On the other hand, Japanese Patent Examined Publication No. 10545/1980 discloses a method of preventing uneven growth by improving local unevenness of concentration. This method is characterized in that a mixing vessel, having openings in each of the upper and lower portions thereof, is sunken in a reactor filled with an aqueous solution of colloid, an aqueous solution of halide and an aqueous solution of silver salt are supplied thereto from the lower opening through respective supply tubes, the reaction solutions are rapidly stirred and mixed by a lower impeller (turbine blades) of the mixing vessel to form silver halide, and the resulting silver halide grains are immediately discharged from the upper opening of the mixing vessel into the aqueous solution of colloid in the reactor by an upper impeller provided above the lower impeller.
Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 92523/1982 discloses a method using the same mixing vessel as above, wherein an aqueous solution of halide and an aqueous solution of silver salt are separately supplied from the lower opening, the reaction solutions, diluted with an aqueous solution of colloid, is rapidly stirred and mixed by the lower impeller and immediately subjected to rapid shearing mixing in the space between the inner wall of the mixing vessel and the blade ends of the impeller and discharged upward, and an apparatus therefor.
However, in the production methods and apparatuses described above, although local unevenness of silver ion and halogen concentrations in the reactor can be perfectly eliminated, such unevenness remains in the mixing vessel; a dense portion is fairly widely distributed particularly in the vicinities of the nozzles via which the aqueous solution of silver salt and that of halide are supplied, in the space under the impeller and in the stirring portion. Further, the silver halide grains supplied with protective colloid to the mixing vessel rapidly grow in different modes in the portions with uneven distribution of concentration. In other words, in these production methods and apparatuses, uneven distribution of concentration is present in the mixing vessel, where grain growth occurs rapidly, which distribution of concentration does not ensure the even growth of silver halide.
With the aim of eliminating such uneven distribution of silver ion and halide ion concentrations by improved mixing, attempts have been made to supply an aqueous solution of silver salt and an aqueous solution of halide to a mixing vessel separate from a reactor and rapidly mixed to form silver halide grains. For example, Japanese Patent O.P.I. Publication No. 37414/1978 and Japanese Patent Examined Publication No. 21045/1973 each disclose a production method wherein an aqueous solution of protective colloid (containing silver halide grains) in the reactor is circulated by a pump from the bottom of the reactor, an aqueous solution of silver salt and an aqueous solution of halide are supplied to a mixing vessel which is provided in the circulatory system, and rapidly mixed therein to grow silver halide grains, and an apparatus therefor. U.S. Pat. No. 3,897,935 discloses a method wherein an aqueous solution of halide and an aqueous solution of silver salt are injected by a pump to a circulatory system of the same embodiment as above. Japanese Patent O.P.I. Publication No. 47397/1978 discloses a production method wherein an aqueous solution of alkali metal halide is first injected to a circulatory system of the same embodiment as above and allowed to diffuse until it becomes uniformly distributed, and subsequently an aqueous solution of silver salt is injected to the circulatory system and mixed with the former aqueous solution, and an apparatus therefor. Although these methods permit separate control in the flow rate of solution from the reactor to the circulatory system and the stirring efficiency of the mixing vessel and allow grain formation with uniform supply rate per previously formed silver halide grain, the problem remains unsolved that silver halide crystals grow rapidly at the injection ports for the aqueous solution of silver salt and the aqueous solution of halide.
In addition, the grains passing a portion of aqueous solution of silver salt localized in the initial stage of mixing, will form so-called silver bodies which are covered with silver ion, while those passing a portion of aqueous solution of halide will form so-called halogen bodies which are covered with halogen ions. The grains undergoing cycles of these processes involve a group of grains having different intergranular latency characteristics, involving differences in properties such as intracrystalline lattice defects, the number of transition lines and appearance of crystals.
Also, solely to avoid surrounding the resulting silver halide grains by an uneven distribution of silver ion and halide ion concentrations, it is possible to grow crystals from previously prepared silver halide grains having the same latent character within the acceptable range, by Ostwald ripening. Such methods include those disclosed in Japanese Patent O.P.I. Publication Nos. 65925/1973, 88017/1976, 153428/1977 and 99751/1987 and J. Col. Int. Sci., 63 (1978) No. 1, p. 16, and P. S. E. 28 (1984), No. 4, p. 137. When Ostwald ripening is practically used, the speed of grain growth to grown grains is high in the small size of the grains dissolved and reprecipitated. The methods described above however, are not practical because of high production cost and poor productivity. The speed of Ostwald ripening is so slow that much time is consumed in growing silver halide grains because the silver halide grains added are not sufficiently smaller than the silver halide grains to be grown.
As a means of forming fine silver halide grains, T. H. James cites the Lippman emulsion as fine grains "The Theory of the Photographic Process", 4th edition, specifying an average size of 0.05 .mu.m. Also, Japanese Patent O.P.I. Publication Nos. 183417/1989 and 183645/1989, W089-06830 and W089-06831 and other publications disclose methods wherein fine silver halide grains are formed in a mixing vessel provided outside the reactor and immediately supplied to the reactor, where crystals are grown (see FIG. 6).
The method disclosed in Japanese Patent O.P.I. Publication No. 183417/1989 is characterized in that an aqueous solution of silver salt, an aqueous solution of colloid and an aqueous solution of halide are simultaneously supplied to a mixing vessel provided outside the reactor to form fine grains, which are immediately supplied to the reactor, where crystals are grown. This method offers very uniform silver halide because the grains during crystal growth do not come in contact with the aqueous solution of silver salt and the aqueous solution of halide.
In this method, based on a crystal growing method by supplying fine grains which has long been known in the photographic industry, the dissolution of fine grain is the rate determining factor, posing a problem of extended production time; however, short time grain growth is possible by adding the grains prepared in the mixing vessel while they remain very fine.
However, as is evident from FIG. 6, grain formation is never instantaneous because the grains are supplied from a mixing vessel provided outside the reactor, though grain formation is faster than with pre-formed fine grains. As is known well from "Photographic Science and Engineering", Vol. 23, No. 2 118 (1979), for instance, the silver halide reaction proceeds too rapidly. As described in this publication, even in the reaction of silver chloride, which is recognized as of relatively slow reaction, at a concentration of as low as about 10.sup.-4 mol/l, the reaction completes itself in a few dozen to a few hundred msec. Therefore, even this method encounters the growth of the fine grains formed. This remains in the range of conventional grain growth from fine grains; more time is required to form grains in comparison with the double jet method. Changes in solubility product, an index of solubility of silver halide, are shown below.
TABLE 1 ______________________________________ Effects of temperature on solubility product Temperature (.degree.C) Silver chloride Silver bromide Silver iodide ______________________________________ 0 1.48 .times. 10.sup.-11 2.03 .times. 10.sup.-14 1.30 .times. 10.sup.-18 10 4.35 .times. 10.sup.-11 8.11 .times. 10.sup.-14 7.65 .times. 10.sup.-18 20 1.14 .times. 10.sup.-10 2.70 .times. 10.sup.-13 3.89 .times. 10.sup.-17 30 2.76 .times. 10.sup.-10 8.50 .times. 10.sup.-13 1.74 .times. 10.sup.-16 40 6.22 .times. 10.sup.-10 2.44 .times. 10.sup.-12 6.95 .times. 10.sup.-16 60 2.57 .times. 10.sup.-9 1.58 .times. 10.sup.-11 8.01 .times. 10.sup.-15 80 8.39 .times. 10.sup.-9 7.46 .times. 10.sup.-11 6.61 .times. 10.sup.-14 100 2.35 .times. 10.sup.-8 2.86 .times. 10.sup.-10 4.01 .times. 10.sup.-13 ______________________________________
This table shows that solubility decreases in the descending order of silver chloride, silver bromide and silver iodide; in any case, solubility decreases as temperature declines. Also, reaction rate decreases in the descending order of silver iodide, silver bromide and silver chloride; silver chloride is slowest.
Therefore, this method requires a long time for producing silver iodobromide for high-sensitivity emulsion.
Furthermore, in regard to sparingly water-soluble salt usable in the photographic field, the following are well known as a sensitivity speck substance for silver halide grains: silver sulfide, gold sulfide and gold and/or silver chalcogenides of sulfur. These substances are known to be formed in the course of sulfur sensitization, gold-sulfur sensitization, gold sensitization, etc., respectively.
Electron microscopic observation of silver sulfide resulting from chemical sensitization of a silver halide emulsion has been reported, for example, by G. C. Furnell, P. B. Flint and D. C. Berch [Journal of Photographic Science, 25, 203 (1977)]. As reported in these reports, the size of fine grains of silver sulfide is very small of the order of several .ANG. to a few dozen .ANG., and they are abundantly present on a silver halide grain surface.
A basic photographic science technical approach to obtainment of fine grains of high-sensitivity emulsion is to increase photon efficiency in the light exposure process. Factors which possibly hamper the increase in the photon efficiency include the presence of competitive electron traps originating from re-bonding, latent image dispersion, structural failure and other origins. Sulfur sensitization and gold-sulfur sensitization are thought to provide the electron capturing center in the light exposure process; therefore, it is a key to chemical sensitizing treatments to adjust the size, position and number of sensitivity specks serving as such sensitization centers. Methods of controlling this position and number are reported or proposed in Japanese Patent O.P.I. Publication Nos. 9344/1986, 40938/1989, 62631/1989, 62632/1989, 74540/1989, 158425/1989, 34/1990 and 298935/1990 and other publications.
However, all these methods aim at limiting the position of formation of the silver sulfide or gold-silver complex sulfide described above; their size and number depend on the limited position (area), with no direct control of the size or number of specks of silver sulfide, gold sulfide or complex thereof.
This is because the grains are as fine as several .ANG. to a few dozen .ANG. as stated above, and because the size and number are significantly affected by the site and area of formation of silver sulfide on the silver halide crystal surface.
On the other hand, some methods have been proposed which are based on techniques different from those of ordinary chemical sensitization.
For example, Japanese Patent O.P.I. Publication No. 93447/1986 describes a sensitizing method wherein not more than 10.sup.-3 mol/mol AgX of fine grains of silver sulfide or gold sulfide are formed at specific points of silver halide crystals, but it gives no specific description about the size or number thereof.
Japanese Patent O.P.I. Publication No. 198443/1990 describes sensitization of silver halide grains by the addition of silver sulfide zol having a fine grain size thereto. However, this publication gives no description of the grain size of the silver sulfide zol obtained, describing nothing other than the luminescence spectrum of the silver sulfide zol. Nor is specified the grain size distribution. As recognized commonly, coloring with colloid grains is largely affected by the size, chemical species and surface condition thereof; it is impossible to specify the size and distribution of colloid grains solely by the spectrum thereof. Therefore, when this method is used, it remains unknown how many specks have been formed on the silver halide crystal because the grain size is unknown.
As stated above, despite the fact that the size and number of fine grains of silver sulfide, gold sulfide, etc. on silver halide crystals are critical factors in the photo sensitizing process, they remain out of control. This is because their size is too small; to date, no one has ever succeeded in controlling the size and number of these grains.
However, with respect to ultrafine grains of gold and/or silver chalcogenide, no records are available even on their size, as stated above, nor has anyone attempted to produce monodispersed grains with narrow distribution. Although the above-mentioned Japanese Patent O.P.I. Publication No. 198443/1990 may be mentioned as a rare case associated with such grains, even this publication does not specify the grain size. Moreover, it describes nothing more than very unclear grain formation conditions, e.g., instantaneous addition of silver nitrate to an aqueous solution of sodium sulfide, followed by addition of an inhibitor. In view of these circumstances, the inventors analyzed various methods which had long been employed for production of silver halide grains, and found a useful combination of essential requirements with more consideration. Specifically, the essential requirements are to integrate or divide the resulting chalcogenide grains to unit aggregates of sensitization specks, to provide solution conditions ensuring atomic arrangement for sensitization specks without causing excess dissolution or decomposition of the unit aggregates, and to prevent excess flocculation of the unit aggregates to ensure stable dispersion, i.e., to use a protective colloid, to control ion concentration during grain formation, and to employ a reaction apparatus capable of controlling temperature and the amount of addition and performing instantaneous mixing. The inventors made investigations as to these items and succeeded in controlling the grain size of ultrafine grains and achieving monodispersion as described above.
When monodispersibility is required for the present invention, it is possible to keep the coefficient of variance below 0.16, as defined as the value obtained by dividing the standard deviation S of grain size by the mean grain size r.
However, with respect to apparatuses and methods for production, a problem of much time requirement remained to be solved in producing perfectly uniform ultrafine grains in a short time as with silver halide grains.